Willow Species Research Articles

Willow traits outperform taxonomy in predicting phytoremediation services

Phytomanagement of contaminated sites can mitigate exposure risks for surrounding populations while providing numerous ecological services. To meet these goals, trait-based models are proposed to guide plant selection. However, this relies on the assumption that plant traits can effectively predict key services and that traits of individual species can be used to predict mean, or community-level traits of a given species assemblage. To critically evaluate these assumptions, we conducted a mesocosm study where three willow species were planted in all possible combinations (1 to 3 species) in contaminated soil under a controlled environment, for 110 days. At the community-level (for every mesocosm), we measured ten functional traits and three phytoremediation services (i.e. phytoextraction, phytostabilization, and translocation factor). We evaluated the differences between observed community-level traits and expectations from traits of the corresponding species grown in monocultures. Then, we compared the predictability of phytoremediation services through species composition and community-level traits. Our results indicate that, despite the short phylogenetic gradient, willow species exhibit distinct and predictable traits within assemblages. Moreover, trait values measured here are comparable to the values retrieved on the TRY database, confirming the potential of global databases to guide trait-based efforts in phytoremediation. Phytoremediation services were not predicted by species composition (mean R2adj = 0.05), but rather well explained by community-level traits (mean R2adj = 0.52). This suggests that models incorporating functional information are better suited to predict and understand phytoremediation services. Phytoextraction was generally correlated to fast-to-intermediate aboveground growth strategies with fast-growing belowground tissues, while translocation factors were associated with slower root growth. Phytostabilization was associated with faster-growing root systems and stress-tolerant capacities from the aboveground tissues. This experiment represent a strong test for trait-based models, given the short phylogenetic and contamination gradients tested. This reinforces the potential of trait-based models in phytoremediation and phytotechnologies more broadly.

Variation in induced responses in volatile and non-volatile metabolites among six willow species: Do willow species share responses to herbivory?

Chemical variation is a critical aspect affecting performance among co-occurring plants. High chemical variation in metabolites with direct effects on insect herbivores supports chemical niche partitioning, and it can reduce the number of herbivores shared by co-occurring plant species. In contrast, low intraspecific variation in metabolites with indirect effects, such as induced volatile organic compounds (VOCs), may improve the attraction of specialist predators or parasitoids as they show high specificity to insect herbivores. We explored whether induced chemical variation following herbivory by various insect herbivores differs between VOCs vs. secondary non-volatile metabolites (non-VOCs) and salicinoids with direct effects on herbivores in six closely related willow species. Willow species identity explained most variation in VOCs (18.4%), secondary non-VOCs (41.1%) and salicinoids (60.7%). The variation explained by the independent effect of the herbivore treatment was higher in VOCs (2.8%) compared to secondary non-VOCs (0.5%) and salicinoids (0.5%). At the level of individual VOCs, willow species formed groups, as some responded similarly to the same herbivores. Most non-VOCs and salicinoids were upregulated by sap-suckers compared to other herbivore treatments and control across the willow species. In contrast, induced responses in non-VOCs and salicinoids to other herbivores largely differed between the willows. Our results suggest that induced responses broadly differ between various types of chemical defences, with VOCs and non-VOCs showing different levels of specificity and similarity across plant species. This may further contribute to flexible plant responses to herbivory and affect how closely related plants share or partition their chemical niches.

Responses of growth and photosynthesis to alkaline stress in three willow species

Investigating differences in resistance to alkaline stress among three willow species can provide a theoretical basis for planting willow in saline soils. Therefore we tested three willow species (Salix matsudana, Salix gordejevii and Salix linearistipularis), already known for their high stress tolerance, to alkaline stress environment at different pH values under hydroponics. Root and leaf dry weight, root water content, leaf water content, chlorophyll content, photosynthesis and chlorophyll fluorescence of three willow cuttings were monitored six times over 15 days under alkaline stress. With the increase in alkaline stress, the water retention capacity of leaves of the three species of willow cuttings was as follows: S. matsudana > S. gordejevii > S. linearistipularis and the water retention capacity of the root system was as follows: S. gordejevii > S. linearistipularis > S. matsudana. The chlorophyll content was significantly reduced, damage symptoms were apparent. The net photosynthetic rate (Pn), rate of transpiration (E), and stomatal conductance (Gs) of the leaves showed a general trend of decreasing, and the intercellular CO2 concentration (Ci) of S. matsudana and S. gordejevii first declined and then tended to level off, while the intercellular CO2 concentration of S. linearistipularis first declined and then increased. The quantum yield and energy allocation ratio of the leaf photosystem II (PSII) reaction centre changed significantly (φPo, Ψo and φEo were obviously suppressed and φDo was promoted). The photosystem II (PSII) reaction centre quantum performance index and driving force showed a clear downwards trend. Based on the results it can be concluded that alkaline stress tolerance of three willow was as follows: S. matsudana > S. gordejevii > S. linearistipularis. However, since the experiment was done on young seedlings, further study at saplings stage is required to revalidate the results.

Biomass yield and metal phytoextraction efficiency of Salix and Populus clones harvested at different rotation lengths in the field experiment

BackgroundPhytoextraction belongs to environmentally well-accepted remediation technologies to remove metals from contaminated soils. Due to long-time requirement, sufficient data for proper phytoextraction evaluation are missing. Four clones of fast-growing trees: two willow species (S1), Salix viminalis L. (Salix schwerinii E.L.Wolf × S. viminalis) × S. viminalis) and (S2)—Salix smithiana (Salix × smithiana Willd.), and two poplar clones (P1), Populus Max-4 (Populus nigra L. × Populus maximowiczii A. Henry) and (P2) Wolterson (P. nigra L.) were cultivated under field conditions at medium-to-high Cd and Pb, and low Zn soil contamination to assess trees’ long-term ability of biomass production and removal of potentially toxic elements (PTEs). The biomass yield and PTE uptake were measured during 8 years of regular growth under three rotation lengths: four harvests following 2-year periods (4 × 2y), two harvests in 4-year periods (2 × 4y), and one harvest representing 8 years of growth (1 × 8y).ResultsIn most cases, the highest annual dry biomass yield was achieved with a 2 × 4y rotation (P1 = 20.9 t ha−1 y−1, S2 = 18.4 t ha−1y−1), and the yield decreased in order 2 × 4y > 1 × 8y > 4 × 2y of harvesting periods. Only clone S1 showed a different pattern. The differences in biomass yield substantially affected the PTE phytoextraction. The greatest amount of Cd and Zn was removed by willow S2, with the highest biomass yield, and the strongest ability to accumulate PTEs. With 2 × 4y rotation, S2 removed a substantial amount of Cd (9.07%) and Zn (3.43%) from the topsoil horizon (0–20 cm) and 5.62% Cd and 2.04% Zn from horizon 20–40 cm; phytoextraction rate was slightly lower for 1 × 8y rotation. The poplar P1 removed the most Pb in the 1 × 8y rotation, but the overall Pb phytoextraction was negligible. The results indicated that lignin and cellulose contents increased, and hemicellulose content decreased with increased concentrations of Cd, Pb and Zn in poplars wood.ConclusionsThe data confirmed that phytoextraction over longer harvest periods offered promising results for removing Cd from medium- to high-level contaminated soils; however, the ability of Pb removal was extremely low. The longer harvest period should be more economically feasible.Graphical

Effects of individual traits vs. trait syndromes on assemblages of various herbivore guilds associated with central European Salix.

Plants employ diverse anti-herbivore defences that can covary to form syndromes consisting of multiple traits. Such syndromes are hypothesized to impact herbivores more than individual defences. We studied 16 species of lowland willows occurring in central Europe and explored if their chemical and physical traits form detectable syndromes. We tested for phylogenetic trends in the syndromes and explored whether three herbivore guilds (i.e., generalist leaf-chewers, specialist leaf-chewers, and gallers) are affected more by the detected syndromes or individual traits. The recovered syndromes showed low phylogenetic signal and were mainly defined by investment in concentration, richness, or uniqueness of structurally related phenolic metabolites. Resource acquisition traits or inducible volatile organic compounds exhibited a limited correlation with the syndromes. Individual traits composing the syndromes showed various correlations to the assemblages of herbivores from the three studied guilds. In turn, we found some support for the hypothesis that defence syndromes are composed of traits that provide defence against various herbivores. However, individual traits rather than trait syndromes explained more variation for all studied herbivore assemblages. The detected negative correlations between various phenolics suggest that investment trade-offs may occur primarily among plant metabolites with shared metabolic pathways that may compete for their precursors. Moreover, several traits characterizing the recovered syndromes play additional roles in willows other than defence from herbivory. Taken together, our findings suggest that the detected syndromes did not solely evolve as an anti-herbivore defence.

Responses of soil and rhizosphere microbial communities to Cd-hyperaccumulating willows and Cd contamination

BackgroundThe pollution of soil by heavy metals, particularly Cd, is constitutes a critical international environmental concern. Willow species are renowned for their efficacy in the phytoremediation of heavy metals owing to their high Cd absorption rate and rapid growth. However, the mechanisms underlying microbial regulation for high- and low-accumulating willow species remain poorly understood. Therefore, we investigated the responses of soil and rhizosphere microbial communities to high- and low-Cd-accumulating willows and Cd contamination. We analyzed soil properties were analyzed in bulk soil (SM) and rhizosphere soil (RM) planted with high-accumulating (H) and low-accumulating (L) willow species.ResultsRhizosphere soil for different willow species had more NH4+ than that of bulk soil, and RM-H soil had more than RM-L had. The available phosphorus content was greater in hyper-accumulated species than it was in lower-accumulated species, especially in RM-H. Genome sequencing of bacterial and fungal communities showed that RM-L exhibited the highest bacterial diversity, whereas RM-H displayed the greatest richness than the other groups. SM-L exhibited the highest diversity and richness of fungal communities. Ralstonia emerged as the predominant bacterium in RM-H, whereas Basidiomycota and Cercozoa were the most enriched fungi in SM-H. Annotation of the N and C metabolism pathways revealed differential patterns: expression levels of NRT2, NarB, nirA, nirD, nrfA, and nosZ were highest in RM-H, demonstrating the effects of NO3-and N on the high accumulation of Cd in RM-H. The annotated genes associated with C metabolism indicated a preference for the tricarboxylic pathway in RM-H, whereas the hydroxypropionate-hydroxybutyrate cycle was implicated in C sequestration in SM-L.ConclusionsThese contribute to elucidation of the mechanism underlying high Cd accumulation in willows, particularly in respect of the roles of microbes and N and C utilization. This will provide valuable insights for repairing polluted soil using N and employing organic acids to improve heavy metal remediation efficiency.

Сезонная динамика и ритмичность роста побегов растений Salix ‘Bullata’

Abstract. The purpose is to identify infradian rhythms in the seasonal dynamics of apical growth of shoots of Salix ‘Bullata’ plants and to establish the features of morphogenesis associated with the existence of endogenous (genetic and hormonal) mechanisms for the occurrence of nonlinear changes, including fluctuations in the growth characteristics of shoots. Methods. A quantitative approach was used to study the dynamics of morphogenesis processes. There was carried out a comparative research of seasonal dynamics of velocities and accelerations of apical and radial growth of Salix ‘Bullata’ plants shoots, which differ in origin and location on the plant. Results. In the seasonal dynamics of the characteristics of apical growth of branching and plant formation shoots of Salix ‘Bullata’ plants there are observed infradian rhythms, similar to those identified for other willow species. These rhythms are most pronounced for the growth accelerations (about 5 maximums within 3.5 months). Seasonal dynamics of apical growth accelerations of two types of shoots are correlated and almost synchronous. Seasonal dynamics of speeds and accelerations of radial growth of the studied types of shoots are similar and differ from the seasonal dynamics of apical growth. The weak dependence of growth characteristics on weather conditions confirms the concept of the endogenous nature of the growth of plant shoots periodicity. Scientific novelty. The results complement the existing information on biological rhythms and processes of morphogenesis in woody plants. The growth of shoots of willows and, possibly, other species of woody plants has an oscillatory character. Seasonal dynamics of apical and radial growth are different. The occurrence of oscillations in the rate of shoot apical growth is mainly due to the processes of apical meristem development itself. Apparently, the oscillatory nature of shoot growth is the result of the superposition of several processes of different nature in the meristem tissues, including division, extension and differentiation of cells, and also the production and distribution of phytohormones. The results can be applied to the use and modification of methods of woody plants nursing – the grafting, the pruning, the rejuvenation and the vegetative propagation.

Revealing the transitory and local effect of zebularine on development and on proteome dynamics of Salix purpurea.

DNA methylation plays major roles in the epigenetic regulation of gene expression, transposon and transcriptional silencing, and DNA repair, with implications in developmental processes and phenotypic plasticity. Relevantly for woody species, DNA methylation constitutes a regulative layer in cell wall dynamics associated with xylogenesis. The use of methyltransferase and/or demethylase inhibitors has been proven informative to shed light on the methylome dynamics behind the regulation of these processes. The present work employs the cytidine analog zebularine to inhibit DNA methyltransferases and induce DNA hypomethylation in Salix purpurea plantlets grown in vitro and in soil. An integrative approach was adopted to highlight the effects of zebularine on proteomic dynamics, revealing age-specific (3 weeks of in vitro culture and 1 month of growth in soil) and tissue-specific (stem and root) effects. After 3 weeks of recovery from zebularine treatment, a decrease of 5-mC levels was observed in different genomic contexts in the roots of explants that were exposed to zebularine, whereas a functionally heterogeneous subset of protein entries was differentially accumulated in stem samples, including entries related to cell wall biosynthesis, tissue morphogenesis, and hormonal regulation. Significant proteomic remodeling was revealed in the development from in vitro to in-soil culture, but no significant changes in 5-mC levels were observed. The identification of tissue-specific proteomic hallmarks in combination with hypomethylating agents provides new insights into the role of DNA methylation and proteome in early plant development in willow species. Proteomic data are available via ProteomeXchange with identifier PXD045653. WGBS data are available under BioProject accession PRJNA889596.

Salix farriae (Salicaceae) – a new sp ecies for FLORA OF the Russia from the Kamchatka Peninsula

A new species of willow for the flora of Russia, Salix farriae C.R. Ball, is presented, found in the north of the Kamchatka Peninsula in 2011. A morphological description of this species and its differences from Salix hastata are given. It is quite probable that Salix farriae was also found on the Commander Islands, but additional full-fledged herbarium collections of plants with catkins are needed.

Phytoremediation Potential of the Coastal Plain Willow Salix caroliniana for Heavy Metals with Emphasis on Mercury

Remediation of heavy metals is a complex, difficult, and expensive exercise and commonly involves relocation of the problem from one place to another (e.g., a contaminated site to a hazardous waste landfill). Alternatively, bioremediation can be used so that metals can be sequestered into the woody portion of the plant, where they are fixed into the organic matter and remain undisturbed for extended periods (e.g., wetland area soils). A species of willow (Salix caroliniana) was investigated to ascertain its potential for use in the bioremediation of mercury. It was found that the plant did bioaccumulate mercury and did so especially in the wood compared to its leaves (8.8 to 1 ratio in the mercury activity; activity of 12,071 to 1164). While there is moderate potential for use in bioremediation, the measured concentration in the wood was quite low (average of 11.91 ppb). The low concentration was caused by the location of the sampled trees in environments that were either pristine or had low soil concentrations of mercury. Furthermore, 90% of the mercury accumulated in the leaves appeared to be adsorbed via atmospheric deposition in southwest Florida where there are minimal sources of anthropogenic mercury, but the region is impacted by Saharan dust, which does contain mercury.

Effects of willow and Sedum alfredii Hance planting patterns on phytoremediation efficiency under AC electric field.

Heavy metal contamination to soil is tricky due to its difficult removal, long retention time, and biomagnified toxicity. The green and low-cost phytoremediation with electric field treatment and planting pattern selection is an emerging and more effective approach to remove heavy metals from soils. In this study, alternating current (AC) electric field-assisted phytoremediation was examined with different planting patterns, i.e., monoculture willow (Salix sp.), monoculture Sedum alfredii Hance, and interplanting of willow and S. alfredii. AC electric field greatly increased phytoremediation efficiency to soil cadmium (Cd) regardless of planting patterns, either single plant species of willow or S. alfredii. The Cd removal capacity of willow and S. alfredii raises apparently under 0.5Vcm-1 AC electric field. Under different planting patterns of AC electric field treatment, Cd accumulation in the whole plant by interplanting was 5.63 times higher than monoculture willow, but only 0.75 times as high as monoculture S. alfredii. The results showed that AC electric field-assisted interplanting of willow and S. alfredii is a promising remediation technique for efficiently clean-up Cd-contaminated soil.

Reproductive Sexual Dimorphisms in Two Willow Species, Salix exigua Nutt. and Salix nigra Marshall

Reproductive Sexual Dimorphisms in Two Willow Species, <i>Salix exigua</i> Nutt. and<i> Salix nigra</i> Marshall

First report of Melampsora ferrinii (pucciniales) on Babylon willow (Salix babylonica) in Iran

As an agroforestry resource willow species (Salix spp.) becoming increasingly important in the protection of riverbanks, recovery of floodable lands, soil bioremediation, biomass production, and wicker furniture manufacture, Babylon willow (Salix babylonica) is widely planted all over the world and is found throughout Iran as part of the rural landscape and is mostly planted for ornamental or bioremediation purposes. Rust fungi in the genus Melampsora usually cause disease on Salix species. Identification of Melampsora species is often complicated because there are few differences in spore morphology and little publicly available comparative sequence data. Willow trees in Iran have been reported to be infected by 15 Melampsora species; however, most of these records are based on morphological characterization. During surveillance activities carried out in June 2007, a rust species was detected on isolated Babylon willow trees in an area in Northern Iran. Both the rust fungus and the host plant were identified on the basis of morphological features and DNA sequencing. Based on a combination of morphology, large subunit (LSU) rDNA sequencing and phylogenetic analyses, our rust specimen was identified as Melampsora ferrinii, which is newly reported from Iran on Salix babylonica. Melampsora ferrinii was originally described from the Americas and this is the first time this rust has been observed on its telial host, Babylon willow, in Asia. The documentation of the rust fungus was performed using both scanning electron microscopy (SEM) and light microscopy. Additionally, an identification key for Melampsora species reported on Salix spp. in Iran is included.

Large effect of phosphate-solubilizing bacteria on the growth and gene expression of Salix spp. at low phosphorus levels.

Phosphorus is one of the most important nutrients required for plant growth and development. However, owing to its low availability in the soil, phosphorus is also one of the most difficult elements for plants to acquire. Phosphorus released into the soil from bedrock quickly becomes unavailable to plants, forming poorly soluble complexes. Phosphate-solubilizing bacteria (PSB) can solubilize unavailable phosphorus-containing compounds into forms in which phosphorus is readily available, thus promoting plant growth. In this study, two willow species, Salix dasyclados cv. Loden and Salix schwerinii × Salix viminalis cv. Tora, were inoculated with two selected bacterial strains, Pantoea agglomerans and Paenibacillus spp., to evaluate the plant growth parameters and changes in gene expression in the presence of different concentrations of tricalcium phosphate: 0 mM (NP), 1 mM (LP), and 2 mM (HP). Inoculation with PSB increased root, shoot and leaf biomass, and for the HP treatment, significant changes in growth patterns were observed. However, the growth responses to plant treatments tested depended on the willow species. Analysis of the leaf transcriptomes of the phosphate-solubilizing bacterium-inoculated plants showed a large variation in gene expression between the two willow species. For the Tora willow species, upregulation of genes was observed, particularly for those involved in pathways related to photosynthesis, and this effect was strongly influenced by bacterial phosphate solubilization. The Loden willow species was characterized by a general downregulation of genes involved in pathway activity that included ion transport, transcription regulation and chromosomes. The results obtained in this study provide an improved understanding of the dynamics of Salix growth and gene expression under the influence of PSB, contributing to an increase in yield and phosphorus-use efficiency.

Phytochemical Screening of Ultrasonic Extracts of Salix Species and Molecular Docking Study of Salix-Derived Bioactive Compounds Targeting Pro-Inflammatory Cytokines.

Willow bark (Salix spp., Salicaceae) is a traditional analgesic and antirheumatic herbal medicine. The aim of this study was to evaluate and compare the phytochemical and antioxidant profiles of leaf and bark extracts of six species of the genus Salix obtained by ultrasound-assisted extraction (UAE) and to examine the inhibitory potential of target bioactive compounds against two inflammatory mediators, tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6), through in silico molecular docking. The total phenolic and flavonoid content of the extracts was estimated using spectrophotometric methods and the antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl (DPPH•) and hydroxyl radical (•OH) scavenging assays. Chemical profiling of extracts was carried out using high-performance liquid chromatography coupled with a diode array detector (HPLC-DAD). Principal component analysis (PCA) was performed to differentiate the sample extracts based on their phytochemical profiles and amounts of target bioactive compounds. Chemical composition varied among the analyzed willow species and also among the plant organs of the same species. The major bioactive compounds of the extracts were salicin, chlorogenic acid, rutin and epicatechin. The extracts exhibited significant DPPH● and ●OH scavenging activities. Results of molecular docking revealed that chlorogenic acid had the highest binding affinity toward TNF-α and IL-6. UAE extracts represent valuable sources of antioxidant and anti-inflammatory compounds.

Distribution of Willows Along Streambanks of Publicly Managed Streams in the Interior Pacific Northwest

Willows are an important component of western riparian zones as their roots stabilize streambanks, their overstory mediates stream temperature, and aquatic biota use their stems and leaves as forage and building material. We evaluated 1894 stream reaches in the Inland Pacific Northwest for the presence of willows and detected one or more willow species at 1 247 (66%) of those reaches. Most stream reaches where willow was present had two or more willow species. We identified 23 unique species, of which 11 were found in 50 or more stream reaches. Across all sites, stream reaches with willow were more likely to be in the southern high-elevation portion of the study area. Distribution models constructed using random forests included a measure of spatial location (latitude, longitude, elevation) as one of the top predictors for 10 of the 11 common species. Only one species included a stream reach-scale attribute (bankfull stream width) as a top predictor, although two species had gradient or percent of the reach forested as a secondary predictor. While spatial location within the study area was a key factor in determining species presence, differences among species resulted in willows being found across the range of landscape and stream reach conditions except where forest cover in the reach was high (> 75%). Given that willows share characteristics such as rapid root growth, rhizomes, flexible stems, and vegetative reproduction, in many situations different species may respond as functional equivalents when it comes to protecting streambanks, providing habitat for fish and wildlife, and promoting stream heterogeneity. Ongoing threats to willows near streams on public lands include grazing, water diversion, mining, and climate change. Each of these threats can be at least partially addressed by identifying stream reaches inhabited by willow and establishing reach-scale management objectives for this taxon.

Plant volatiles and priority effects interactively determined initial community assembly of arthropods on multiple willow species.

Plant traits, which are often species specific, can serve as environmental filtering for community assembly on plants. At the same time, the species identity of the initially colonizing arthropods would vary between plant individuals, which would subsequently influence colonizing arthropods and community development in the later stages. However, it remains unclear whether interindividual divergence due to priority effects is equally important as plant trait-specific environmental filtering in the initial stages. In this study, we propose that plant volatile organic compounds (PVOCs) may play a crucial role as an environmental filter in the initial stages of community assembly, which can prevent the community assembly process from being purely stochastic. To test this hypothesis, we conducted short term but highly frequent monitoring (19 observations over 9 days) of arthropod community assembly on intact individuals of six willow species in a common garden. PVOC compositions were analyzed before starting the experiment and compared with arthropod compositions occurring on Days 1-2 of the experiment (earliest colonizer community) and those occurring on Days 8-9 of the experiment (subsequent colonizer community). Unintentionally, deer herbivory also occurred at night of Day 2. Distance-based statistics demonstrated that PVOC compositions were plant species specific, but neither the earliest colonizer nor the subsequent colonizer community composition could be explained by plant species identity. Rather, Procrustes analysis showed that both the PVOC composition and that of the earliest colonizer community can be used to explain the subsequent colonizer community. In addition, the linkage between PVOCs and the subsequent colonizer community was stronger on individuals with deer herbivory. These findings indicate that PVOCs have widespread effects on initial community assembly, as well as priority effects brought on by stochastic immigration, and that plant species identity only has weak and indirect effects on the actual composition of the community.

Pan-Transcriptome Analysis of Willow Species from Diverse Geographic Distributions

Willows, in the genus Salix, are widespread on the earth with significant ecological and economic values for humans. Although about 500 Salix species have been estimated, the genomic foundation of their adaptations to environments with diverse stresses has been underexplored. Here, we applied a pan-transcriptome approach to investigate the phylogenetic relationships and genetic variations among 16 willow species. A pan-transcriptome of 29,668 gene families was assembled, 69% of which exhibited presence/absence variation across the analyzed species. In comparison to core genes present in all species, shell gene families absent in at least one species were enriched with genes in pathways of signaling transduction and response to stimuli, suggesting their functions in the interaction with diverse environmental factors. A phylogenetic tree of 16 willow species was constructed with high confidence based on 870 single-copy orthologous genes, providing detailed evolutionary relationships of willow sections. The willow species were further assigned into four species clusters using the gene numbers in each family. The diversity of gene family size and gene expression levels among the willow species are closely associated with their geographical distributions. The gene family members involved in DNA repair and cellular response to DNA damage stimuli were expanded in willow species from high-altitude regions in southwestern China, which may contribute to their tolerance to ultraviolet radiation stress. Our study generates a comprehensive pan-transcriptome resource for a large set of Salix species and provides insights into the adaptations of willows to diverse environments, which will be valuable for comparative analysis with other related woody and herbaceous plants.

Assessment of the capability of cadmium accumulation and translocation among 31 willows: four patterns of willow biomass variation response to cadmium.

Cadmium (Cd) pollution threatens food security and the environment. Willow species (Salix, Salicaceae) exhibit a remarkable potential to restore Cd-polluted sites due to their high biomass production and high Cd accumulation capacities. This study examined the Cd accumulation and tolerance in 31 genotypes of shrub willow in hydroponic conditions at varying Cd levels (0μM Cd, 5μM Cd, and 20μM Cd). The root, stem, and leaf biomass of 31 shrub willow genotypes showed significant differences to Cd exposure. Among 31 willow genotypes, four patterns of biomass variation response to Cd were identified: insensitive to Cd; growth inhibition due to excessive Cd supply (high Cd inhibition); low Cd causing inhibited growth, whereas high Cd leading to increased biomass (U-shape); and growth increment with excessive Cd exposure (high Cd induction). The genotypes belonging to the "insensitive to Cd" and/or "high Cd induction" were candidates for the utilization of phytoremediation. Based on the analysis of Cd accumulation of 31 shrub willow genotypes at high and low Cd levels, genotypes 2372, 51-3, and 1052 obtained from a cross between S. albertii and S. argyracea grew well and accumulated relatively more Cd levels than other genotypes. In addition, for Cd-treated seedlings, root Cd accumulation was positively correlated with shoot Cd accumulation and total Cd uptake, demonstrating that Cd accumulation in roots could serve as a biomarker for evaluating the Cd extraction capacity of willows, especially in hydroponics screening. The results of this study screened out willow genotypes with high Cd uptake and translocation capacities, which will provide valuable approaches for restoring Cd-contaminated soils with willows.

Coppice growth traits in sand dune willow (Salix cordata) and comparisons with two common North American shrub willows used for biomass production

Among eight native North American willow species examined for coppice growth performance, the sand dune willow, Salix cordata Michx. (COR) has proved to be one of the most productive and useful for woody biomass production. In a common garden study, clones from the eastern and southwestern regions of Ontario, Canada, outperformed those from northern populations. Geographic region, populations within regions, and clones within populations all accounted for large portions of total variation in coppice growth traits that were compared with two other willows commonly used for biomass production, S. discolor Muhl. and S. eriocephala Muhl. Each species varied significantly in coppice structure and growth. The number of coppice stems per plant (coppice stem number) showed strong positive relationships to stem dry mass in 1- and 3-year-old coppices. Female COR plants were often slightly larger than male plants in this dioecious species, but these differences were not statistically significant, and provided no evidence of a reproductive cost expressed as a differential reduction in subsequent vegetative growth in female vs. male plants. This indicated that a zero sum model in which reproductive and vegetative structures compete for assimilates was not supported by our results. In all three species, clonal selection for growth and commercial biomass production, coppice stem number (not plant sex, longest stem length or corresponding basal stem diameter) was the best criteria for selecting superior willow clones.